There are different power supply modes in different regions; normally, the input of a LED drive power supply in the lighting field comprises high-voltage power frequency AC (namely municipal power), low-voltage DC, high-voltage DC, low-voltage high frequency AC (such as output from electronic transformer) and so on. However, the output of the LED drive power supply is mainly represented by a constant-current source that can change voltage in accommodation with variation to LED forward voltage drop value.
Currently, there are two applicable drive modes: One is constant-voltage source for numerous constant-current sources with each constant-current source providing power supply to each LED independently. Such mode features in flexible combination, and any fault to one LED will not affect operation of other LEDs. Nevertheless, its cost is relatively higher. Another one is a direct constant-current power supply with LEDs in series or parallel connection for operation. It has such advantage as relatively low cost despite of poor flexibility. Furthermore, it is also expected to eliminate fault to one certain LED while ensuring operation of other LEDs. The two modes are in co-existence within a certain period of time. A power supply of multi constant-current output is probably the mainstream trend in the future owing to its low cost and excellent performance.
To accommodate different supply voltages, Chinese Patent Application No. 201310110690.1 discloses a LED drive device that can change the structure of series or parallel connection as per dynamic variation to input voltage to drive LED matrix. It comprises the following items: an input voltage sensing module used to compare the input voltage with different voltage range as preset, and output an input voltage status signal; a control module for the circuit structure in series or parallel connection used to output corresponding control signals to switch combination and constant-current sources available for dynamic change of total current according to input voltage status signals; a switch combination available for change of series and parallel structure of a LED matrix, which can change series and parallel structures of the LED matrix according to different control signal status; a constant-current source available for dynamic change of total current; it is available for dynamic change of total current of the constant-current source according to different control signal statuses in accommodation with the switch combination to maintain the current on the single series LED route within a constant current range. This has effectively eliminated bright and dark areas on LED light fixture, and further minimized strobe-flash to LED light fixture in case of conventional drive under Low DC voltage.
Chinese Patent Application No. 201310218352.X discloses a full voltage LED light in series and parallel connection, comprising at least two LED units, namely a LED drive circuit connected to each LED unit and a control circuit in series and parallel connection; wherein, the control circuit in series and parallel connection comprises at least a positive-pole change-over switch, a negative-pole change-over switch and a 1st voltage detection and control circuit; furthermore, each positive-pole and negative-pole change-over switch is correspondingly connected between positive and negative poles of each LED unit; the 1st voltage detection and control circuit will control ON/OFF of each positive-pole and negative-pole switch to realize mutual parallel connection of each LED unit once current low voltage is detected. On the contrary, it will make each LED unit be in series connection in case of high voltage; as total cut-in voltage is to be increased to the fully acceptable high voltage, each LED unit in series connection is unlikely to be burnt once lighted.
The problem with flexible compatibility with power factor and THD is to be settled despite of the fact that aforesaid prior arts can change series and parallel connection mode.